1. Field
The present invention relates to an apparatus for light intensity adjustment mountable on an automotive camera, comprising at least one first linear polarizing filter, at least one second linear polarizing filter, the first linear polarizing filter and the second polarizing filter are arrangeable in front of a lens unit of an automotive camera such that incident light passes through the at least one first linear polarizing filter and the at least one second linear polarizing filter before entering the lens unit of the automotive camera, and at least one actuator for moving the at least one first linear polarizing filter and/or the at least one second linear polarizing filter with respect to each other such that the intensity of the emergent light is adjustable. Further the present invention relates to an automotive camera with such an apparatus and a vehicle comprising such an automotive camera and/or such an apparatus.
2. Description of Related Art
Cameras are getting increasingly popular for automotive applications: Back-up assist, parking assist, side or interior mirror replacement, mirror blind spot monitoring, lane change assist, to name a few. One of the main components of a camera used for these applications is an image sensor. The image sensor consists of a plurality of pixels, light sensitive elements that collect incident light projected by the lens unit, and transform it into an electrical signal. This signal can be read out from the pixel and stored, transmitted to the display unit for viewing by a driver, used for object detection or recognition, etc.
Pixels are read out with the predefined frequency, called frame rate. For most automotive applications an adequate frame rate is in the range of 15 to 60 frames per second (fps). Some image sensors allow selection of a frame rate at system start up, however, typically, once selected, the frame rate cannot be changed without re-initialization of the imager. Theoretically, a pixel can collect the light between the two consequent read-outs. Thus, for example, if the imager has been initialized to operate at 30 fps, the pixel accumulates the light within 33 ms. However, the lighting conditions under which an automotive camera is supposed to operate, can vary significantly: From 1 Lux illumination at night to 20,000 Lux on a sunny day. And while for low light conditions 33 ms integration time might be adequate, it may be too long for high light conditions. In this case the pixel will be overexposed, i.e., the electrical signal output by the pixel will be driven to its maximum regardless of small variations in object brightness.
In order to overcome this problem, image sensors allow variable integration time while maintaining fixed frame rate. This is achieved by changing time between pixel reset and read out. Whatever charge has been accumulated by the pixel before the reset occurs, it is eliminated. This part of the frame period is often referred to as “closed shutter”, a term coming from film photography where mechanical shutter was used to adjust exposure time. Thus, integration time is defined not by the period between two consecutive frames, or in other words, between two consecutive pixel read outs, but by the period between pixel reset and pixel read out instead. This part of the frame period is called “open shutter”. In order to reduce the amount of light transformed into electrical signal, time between pixel reset and read out is reduced by moving reset as close to read out as necessary. Moving reset towards previous frame read out increases the integration time. Obviously, maximum integration time is limited by the time between two consecutive frames read outs, which in the above example is 33 ms.
However, while solving the problem of imager adaptation to different lighting conditions, variable integration time causes a new problem, which is often referred to as stroboscopic effect. Modern light sources, for example light emitting diodes (LED) often illuminate the scene not continuously, but in short pulses instead. This is because the intensity of light cannot be controlled efficiently by changing the voltage; instead pulse width modulation (PWM) is used. In this case the LED is turned on for several milliseconds, and then switched off for another several milliseconds. By changing the ratio between on and off periods, the overall intensity of light can be adjusted.
Moreover, some devices require that the light is emitted in short flashes, for example emergency vehicle lighting used to convey to other road users the urgency of their journey, or provide warning of hazard when stationary. Strobe lights or LEDs are increasingly popular for those devices. Duration of flashes may be as short as hundreds of microseconds, and their frequency as low as a few Hertz.
In these cases duration of flash emitted by a light source is significantly shorter than the part of frame period corresponding to the open shutter. If the flash occurs when the shutter is closed, the flash cannot be captured by the imager. Thus, the frame in the video stream will appear as if there were no flash at all.